<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
<head>
<!-- Copyright 1997 The Open Group, All Rights Reserved -->
<title>msgsnd</title>
</head><body bgcolor=white>
<center>
<font size=2>
The Single UNIX &reg; Specification, Version 2<br>
Copyright &copy; 1997 The Open Group

</font></center><hr size=2 noshade>
<h4><a name = "tag_000_007_826">&nbsp;</a>NAME</h4><blockquote>
msgsnd - message send operation
</blockquote><h4><a name = "tag_000_007_827">&nbsp;</a>SYNOPSIS</h4><blockquote>
<pre><code>

#include &lt;<a href="sysmsg.h.html">sys/msg.h</a>&gt;

int msgsnd(int <i>msqid</i>, const void *<i>msgp</i>, size_t <i>msgsz</i>, int <i>msgflg</i>);
</code>
</pre>
</blockquote><h4><a name = "tag_000_007_828">&nbsp;</a>DESCRIPTION</h4><blockquote>
The
<i>msgsnd()</i>
function is used to send a message to the queue
associated with the message queue identifier specified by
<i>msqid</i>.
<p>
The argument
<i>msgp</i>
points to a user-defined buffer that must contain
first a field of type
<b>long int</b>
that will specify the type of the message,
and then a data portion that will hold the data bytes of the message.
The structure below is an example of what
this user-defined buffer might look like:
<pre>
<code>
struct mymsg {
    long int    mtype;       /* message type */
    char        mtext[1];    /* message text */
}
</code>
</pre>
The structure member
<b>mtype</b>
is a non-zero positive type <b>long int</b> that can be used
by the receiving process for message selection.
<p>
The structure member
<b>mtext</b>
is any text of length
<i>msgsz</i>
bytes.
The argument
<i>msgsz</i>
can range from 0 to a system-imposed maximum.
<p>
The argument
<i>msgflg</i>
specifies the action to be taken if one or more of the following are true:
<ul>
<p>
<li>
The number of bytes already on the queue is equal to
<b>msg_qbytes</b>,
see
<i><a href="sysmsg.h.html">&lt;sys/msg.h&gt;</a></i>.
<p>
<li>
The total number of messages on all queues
system-wide is equal to the system-imposed limit.
<p>
</ul>
<p>
These actions are as follows:
<ul>
<p>
<li>
If (<i>msgflg</i> &amp; IPC_NOWAIT)
is non-zero, the message will not be
sent and the calling thread will return immediately.
<p>
<li>
If (<i>msgflg</i> &amp; IPC_NOWAIT)
is 0, the calling thread will suspend execution until
one of the following occurs:
<ul>
<p>
<li>
The condition responsible for the suspension no longer exists,
in which case the message is sent.
<p>
<li>
The message queue identifier
<i>msqid</i>
is removed from the system; when this occurs,
<i>errno</i>
is set equal to [EIDRM] and -1 is returned.
<p>
<li>
The calling thread receives a signal that is to be caught;
in this case the message is not sent
and the calling thread resumes execution in the manner
prescribed in
<i><a href="sigaction.html">sigaction()</a></i>.
<p>
</ul>
<p>
</ul>
<p>
Upon successful completion, the following actions are taken
with respect to the data structure associated with
<i>msqid</i>,
see
<i><a href="sysmsg.h.html">&lt;sys/msg.h&gt;</a></i>:
<ul>
<p>
<li>
<b>msg_qnum</b>
is incremented by 1.
<p>
<li>
<b>msg_lspid</b>
is set equal to the process ID of the calling process.
<p>
<li>
<b>msg_stime</b>
is set equal to the current time.
<p>
</ul>
<br>
</blockquote><h4><a name = "tag_000_007_829">&nbsp;</a>RETURN VALUE</h4><blockquote>
Upon successful completion,
<i>msgsnd()</i>
returns 0.  Otherwise, no message will be sent,
<i>msgsnd()</i>
will return -1 and
<i>errno</i>
will be set to indicate the error.
</blockquote><h4><a name = "tag_000_007_830">&nbsp;</a>ERRORS</h4><blockquote>
The
<i>msgsnd()</i>
function will fail if:
<dl compact>

<dt>[EACCES]<dd>
Operation permission is denied to the calling process.
See
<a href="ipc.html">IPC</a>.

<dt>[EAGAIN]<dd>
The message cannot be sent for one of the reasons cited above and
(<i>msgflg</i> &amp; IPC_NOWAIT) is non-zero.

<dt>[EIDRM]<dd>
The message queue identifier
<i>msgid</i>
is removed from the system.

<dt>[EINTR]<dd>
The
<i>msgsnd()</i>
function was interrupted by a signal.

<dt>[EINVAL]<dd>
The value of
<i>msqid</i>
is not a valid message queue identifier,
or the value of
<b>mtype</b>
is less than 1; or the value of
<i>msgsz</i>
is less than 0 or greater than the system-imposed limit.

</dl>
</blockquote><h4><a name = "tag_000_007_831">&nbsp;</a>EXAMPLES</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_007_832">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
The POSIX Realtime Extension defines alternative interfaces for interprocess communication.
Application developers who need to use IPC should design their
applications so that modules using the IPC routines described in
<a href="ipc.html">IPC</a>
can be easily modified to use the alternative interfaces.
</blockquote><h4><a name = "tag_000_007_833">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_007_834">&nbsp;</a>SEE ALSO</h4><blockquote>
<i><a href="mq_close.html">mq_close()</a></i>,
<i><a href="mq_getattr.html">mq_getattr()</a></i>,
<i><a href="mq_notify.html">mq_notify()</a></i>,
<i><a href="mq_open.html">mq_open()</a></i>,
<i><a href="mq_receive.html">mq_receive()</a></i>,
<i><a href="mq_send.html">mq_send()</a></i>,
<i><a href="mq_setattr.html">mq_setattr()</a></i>,
<i><a href="mq_unlink.html">mq_unlink()</a></i>,
<i><a href="msgctl.html">msgctl()</a></i>,
<i><a href="msgget.html">msgget()</a></i>,
<i><a href="msgrcv.html">msgrcv()</a></i>,
<i><a href="sigaction.html">sigaction()</a></i>,
<i><a href="sysmsg.h.html">&lt;sys/msg.h&gt;</a></i>,
<a href="ipc.html">IPC</a>.
</blockquote><h4>DERIVATION</h4><blockquote>
Derived from Issue 2 of the SVID.
</blockquote><hr size=2 noshade>
<center><font size=2>
UNIX &reg; is a registered Trademark of The Open Group.<br>
Copyright &copy; 1997 The Open Group
<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]

</font></center><hr size=2 noshade>
</body></html>

